The field of solar thermal energy is well established in the prior art. In one form, solar collectors and receivers are arranged into so-called concentrating solar power (CSP) systems. CSP systems traditionally involve large arrays of mirrors that reflect the sun to a focal point or line, where the intense concentrated sunlight is used to heat fluid. Systems that focus light to a point are generally known as “power tower” or just “tower” systems, in reference to the tall tower that supports the energy collector at the focus of the array of mirrors. Systems that focus light to a line are generally known as “trough” systems, since the long linear focusing mirrors resemble large troughs.
In some systems, this fluid (typically water, which is turned into steam) is used directly to produce electric energy. This is often accomplished by using steam to spin a turbine which drives a generator. In other systems, an intermediate fluid is heated, and then used to produce energy, by likewise using the hot fluid to produce steam. Other approaches are also known, such as using the heat to produce very hot air which drives a gas turbine.
In some cases, the intermediate fluid is stored in a tank for use later. By way of example, there are commercial systems in operation today that use molten salt as the heat transfer fluid, and store the heated salt in large tanks, until such time as energy production is desired, thus providing for the ability to store the collected solar energy for use at a later time.
Another less commonly used approach to generate electricity from solar thermal energy is called a Solar Updraft Tower. In its traditional form, it comprises a chimney in fluid communication with a greenhouse that surrounds it. The greenhouse is open around its outside perimeter, and serves as an air inlet. Sunlight warms the ground under the greenhouse, which causes air to flow towards and then up the chimney, driven by natural convection. Wind turbines placed at the base of or inside the chimney are used to generate electricity.
There is one well-publicized example of storing the solar energy directly in a solid. The Australian company Graphite Energy has demonstrated a system that uses a field of mirrors to heat a large central block of graphite, weighing many tons, that is located at the top of a tall tower. The mirrors focus on the bottom of the block of graphite. The large thermal mass of the graphite block means that it can be used to store energy just as the tanks of molten salt do at other CSP plants. Pipes run through the graphite block to allow the energy to be transferred into a heat transfer fluid or working fluid (which flows through the pipes) when energy production is desired.
Likewise, residential- and commercial-scale solar thermal energy systems are also well known, most often used for heating water. Residential systems typically include a large, well-insulated tank to hold the heated water, since, while energy collection is mostly during the middle of the day, the heated water is generally used mostly in the morning. This tank can add substantially to the cost of the system.
In general, presently marketed commercial and residential systems are based on heating a liquid. If storage is desired, the heated liquid is then stored in a tank.
Unfortunately, storage tanks, and the associated plumbing, valves, pumps, and controls add cost to both residential/commercial and utility-scale systems. Further, these pumps, valves, tanks, and plumbing can be expensive, especially in utility-scale systems where the temperatures are very high, and the materials somewhat exotic. (A pump capable of reliably pumping molten salt at 540° C. is not a common item, and many of the tanks and tubes for use with molten salt have to be made of expensive stainless steel that is capable of withstanding high temperatures.)
CSP using molten salt makes use of this fluid as both a heat transfer fluid, which conveys the heat energy away from the receiver, and as a storage material, which stores the heat energy for later use. While this may seem an efficient use of a resource, combining these functions into a common material ends up making compromises in the performance of both functions.
It would be beneficial to have a system in which distinct materials, that are specifically chosen for each role, perform the heat transfer and energy storage functions, allowing each to perform its role more efficiently.
It would likewise be desirable to be able to store energy without having to handle liquids. The Graphite Energy system has addressed this by providing a very large block of graphite. However, such an enormous block of graphite is itself an exotic item, and there is substantial expense involved in erecting a tower capable of supporting the block, and in fabricating the block with embedded plumbing to allow heat extraction. A further drawback of this system is that it is not extensible to large solar fields, since only mirrors that are close to the graphite block have a clear view of the bottom of the block.
It would be beneficial to have a system that provided the advantages of solid state energy storage, without requiring the solid material to be in an exotic form or placed so high in the air.
One attempt to provide an alternative system has been to embed pipes into blocks of graphite or concrete located on the ground, such as the systems from NEST (pipes in blocks of concrete) or from RAPS Systems (pipes in blocks of graphite). In these systems, heat collection takes place away from the storage system and the heat is piped to the storage medium. Such a system naturally incurs losses during heat absorption, transport, injection, and extraction, and there is significant cost in deploying a system whose only function is energy storage (as compared to systems like molten salt CSP that make dual use of the molten salt as a heat transfer fluid and an energy storage medium).
Other prior art systems have been proposed that use bundles of optical fibers to transport concentrated sunlight across a distance to a remote location where the bundle comes together with other bundles to heat a storage material, such as molten salt. However, these fiber bundles are very expensive. A solution that could eliminate the long-distance transportation of concentrated sunlight could help to reduce this cost.
Other prior art systems have taught the technology of pebble beds for energy storage. In a typical system, energy is collected and transferred to a gaseous heat transfer fluid, typically air (often pressurized) or perhaps nitrogen. The heated air is then forced through the pebble bed, heating the pebbles. Later, when energy extraction is desired, ambient air is forced through the pebble bed, getting hot in the process. The heated air can then be used to turn a gas turbine, create steam, or drive a Stirling engine.
However, pebble bed systems tend to be difficult to work with. Among other things, such systems generally operate at pressures well above ambient, and there is substantial energy loss in forcing dense, pressurized air through the pebbles, and there is a fair amount of cost in the pressure vessel and in the plumbing to deliver the pressurized air to the bed. A more efficient approach to storing and retrieving solar energy from such solid material would be desirable.
Likewise, for home and commercial systems, it would be desirable to have a system that could provide hot water on demand without requiring extra plumbing and large tanks.
If the heat that is produced and stored is of sufficiently high temperature, it can also be used efficiently to generate electricity, instead of or in addition to being used simply for heating. One example of a useful generator to pair with such a system is a Stirling engine. Typical residential- and commercial-scale solar thermal collectors do not heat their water to sufficiently high temperatures to be used for cost-effective electricity production. Efficient production would require heating the water to temperatures over 100° C., meaning that a boiler or high-pressure water system would be required, which would add too much to the cost.
It would thus be beneficial to have a residential- or commercial-scale solar thermal energy collection system that could produce and store heat at temperatures in excess of 100° C. in a medium other than water.
Some companies (e.g. Glasspoint) are using solar thermal energy to aid in the extraction of fossil fuels. The use of heat to help extract fossil fuels from the ground is called Enhanced Oil Recovery (EOR). However, existing systems only operate when the sun is out. It would be beneficial to be able to provide heat on demand for these operations.
Some operations, like oil recovery, and off-grid mining, desire to have portable energy generation. Currently available solar energy devices tend to not be portable. It would be desirable to have solar energy generation and storage that is portable, to help meet these needs.